1. Field of the Invention
This invention pertains to a method for producing a holographic optical element (HOE) that is comprised of a primary hologram (as defined herein) and a complementary hologram (as defined herein). The element is especially useful as a security device. The invention also pertains to methods for establishing the authenticity of the HOE when used as a security hologram as well as the HOE itself.
2. Description of Related Art
A contact copy of a typical hologram, e.g., a master hologram, can be made in a volume photosensitive recording material, such as a photopolymer holographic recording film (HRF), by placing the HRF either in contact with or optically coupled to the primary hologram in such a way that a beam of coherent light illuminates the hologram, causing an image to play back and illuminate the HRF acting as an object wave. At the same time, the beam of coherent light illuminates the recording film to act as a reference wave. Interaction of the object and reference waves produces an interference pattern that is recorded in the HRF to produce the contact copy of the master hologram. FIG. 1 shows contact copying of a reflection hologram where the object wave is designated as SIG and the reference wave is designated as REF. In contact copying a reflection hologram, the beam of coherent light first strikes the HRF (1) and then the master hologram (5) (original, to be copied), causing playback from the master hologram to strike the HRF in the same region and thus generate an interference pattern which results in the hologram being recorded in the HRF. Contact copying includes both rigorous contact copying, where the distance between hologram (5) and recording medium (1) is zero, and xe2x80x9cnear-contactxe2x80x9d copying, where the distance between hologram (5) and recording medium (1) is small, generally a centimeter or less. Such contact copying is well known in the art and is documented in references such as xe2x80x9cHologram Copyingxe2x80x9d, by William T. Rhodes, or The Holography Handbook, edited by H. J. Caufield, Academic Press (1979), pages 373-8.
Holograms are useful in security applications such as, for example, providing a way of establishing authenticity of documents and/or other significant articles, such as credit cards. Use of holograms in security applications is attractive since holographic imaging is not widely known nor easily practiced by would-be counterfeiters and also because it can easily provide a pleasing, aesthetically interesting image. However, because contact copying can be employed to copy any master hologram, these holograms have limited use in security applications and may be subject to counterfeiting. A method of counterfeiting holograms is to use a valid hologram as a master and to use holographic contact copying techniques to make counterfeit holograms which are copies of the valid hologram (master) but which appear to be valid, since the copied counterfeit holograms are indistinguishable or nearly indistinguishable from the master. There is a need in security applications for holographic elements and associated processes for producing these elements that cannot be copied. The present invention provides a solution to this critical need.
This invention provides a method for producing a holographic optical element (HOE) comprised of at least one primary hologram and at least one complementary hologram, such that the holographic optical element is not copyable in total and which consequently cannot be counterfeited using holographic techniques presently known to the art. This invention also provides the holographic optical element, which is useful especially for security applications. It also provides methods for validating the authenticity of the holographic optical element of this invention.
In one embodiment, the invention is a method for producing a holographic optical element useful as a security device comprising:
(a) placing a photosensitive layer adjacent to a surface-relief hologram in a manner such that the photosensitive layer and the surface-relief hologram are in a relationship selected from the group consisting of 1) direct contact and 2) separated by one or more media with each medium having refractive index greater than 1;
(b) exposing the photosensitive layer and the surface-relief hologram to a coherent light beam to record the surface-relief hologram in the photosensitive layer as reflection holograms comprising at least one primary hologram and at least one complementary hologram; and
(c) separating the surface-relief hologram from the imaged photosensitive layer to obtain the volume holographic optical element.
In another embodiment, the invention is a method for establishing the authenticity of the inventive holographic optical element comprising the steps of:
(a) illuminating the holographic optical element with white light at an angle xcex8 measured with respect to a normal line to a surface of the holographic optical element and simultaneously viewing the holographic optical element along the normal line to thereby see a holographic image having a first color, wherein ¦xcex8¦ is approximately in the range of 15 degrees to 55 degrees;
(b) illuminating the holographic optical element with white light at approximately an angle xcex4=xe2x88x92xcex8xc2x115xc2x0 (angle xcex4 equals minus xcex8 plus or minus 15xc2x0) measured with respect to a normal line to the surface of the holographic optical element and simultaneously viewing the holographic optical element along the normal line to ascertain if a holographic image is observed having a second color that is characteristic of a wavelength that is longer than that for the first color; and
(c) establishing the holographic optical element to be authentic only if a holographic image of the first color is observed in step (a) and a holographic image of the second color is observed in step (b).
In another embodiment, the invention is another method for establishing the authenticity of the inventive holographic optical element comprising the steps of:
(a) illuminating a first surface of the holographic optical element with white light at an angle xcex8 measured with respect to a normal line to a center plane of the holographic optical element and simultaneously viewing the first surface of the holographic optical element along the normal line to thereby see a holographic image having a first color, wherein |xcex8| is approximately in the range of 15 to 55 degrees (xc2x0);
(b) rotating the holographic optical element 180xc2x0 about an axis in the center plane of the holographic optical element with continued illumination such that illumination from step (a) is now incident upon a second surface, and simultaneously viewing the second surface of the holographic optical element along the normal line to ascertain if a holographic image is observed having a second color that is characteristic of a wavelength that is longer than that for the first color; and
(c) establishing the holographic optical element to be authentic only if a holographic image of the first color is observed in step (a) and a holographic image of the second color is observed in step (b).
The first surface is the side of the holographic optical element from which imaging by exposure to a coherent light source was done, and the second surface is the side opposite to the first surface (side opposite to the side nearest the coherent light source).
In the aforementioned embodiments for authenticating a hologram, it is also possible to illuminate the HOE with white light at 0xc2x0 (along a normal line) and to observe the hologram to ascertain its color along an angle that is not along the normal line.